Biology of Fabia subquadrata— Pearce 
13 
the xa.et.hM crabs which he studied, and Hiatt 
(1948:157) who reports that in the grapsoid, 
P, crassipes , the legs are first removed, then the 
abdomen. 
As soon as the posterior pair of appendages 
are free the animal then apparently uses them 
to exert pressure against the old integument in 
such a manner as to push the rest of the body 
free of the exuvia or cast. By this time the crab 
has moved far enough posteriorly within the cast 
to allow 'the more anterior pereiopods and 
mouth parts to be freed. The former can then 
be pulled into the area vacated by the cephalo- 
tfaorax proper. This description is true of all the 
pre- and posthard crabs which were observed. 
In the case of the Stage I crab it is more difficult 
to determine the manner in which the anterior 
appendages are freed since the exoskeleton of 
this stage is completely opaque. 
The active phase of exuviation varied between 
15 and 45 minutes, with the average time being 
20 minutes. The larger crabs (greater than 10 
mm in carapace width) took, on an average, 
somewhat longer. There were exceptions, how- 
ever. The longest period observed was taken by 
a Stage III female 5.7 mm in carapace width. 
There was little difference in the average time 
required by hard Stage I or pre- and posthard 
crabs. 
In only 2 out of 61 closely observed moult- 
ings was F. subquadrata seen to moult during 
the daylight hours. This might seem surprising 
in view of the fact that mussel crabs are rarely 
in a photic situation and thus darkness would 
not be of protective advantage during the cru- 
cial period of moulting. However, since most of 
the freeliving brachyurans do moult at night 
( Broekhuysen, 1941; McKay, 1942; Hiatt, 1948; 
and Knudsen, 1957), it can be hypothesized 
that F. subquadrata retains an inherited mechan- 
ism involving the inhibition of moulting by 
light. As noted by these authors such a mechan- 
ism would have obvious adaptive advantages to 
freeliving forms, but it would be of little sig- 
nificance to a symbiotic crab living in a non- 
photic situation. 
The length of time required for the maximum 
postexuvial expansion to occur was not deter- 
mined in every observed moult. However, in the 
cases in which a crab was measured more than 
once following ecdysis no measurable expansion 
was noted after the first postmouk measurement 
was made. The first postmouk measurement was 
routinely taken 30 minutes following the com- 
pletion of exuviation. This implies that the crab 
expands to its postexuvial dimensions during 
and immediately following ecdysis, with little or 
no increase occurring over an extended period 
following ecdysis. This agrees favorably with the 
minimum time required for the final expansion 
of the freeliving xanthid crabs (Knudsen, 1957: 
141 ) .. The latter required from 30 minutes to 
2 hours. 
The degree of postexuvial size increment in 
F. subquadrata varies not only with the stage at 
which the moult occurs but also to some extent 
between individuals of the same stage. Prehard 
crabs moulting into new prehard instars had an 
average increase of 16.5%, with the smaller 
crabs, i.e., the second or thrid postplanktonic 
instars, increasing as much as 20%. Similar 
increases have been found in the early instars 
of other brachyuran species (Olmstead and 
Baumberger, 1923; Broekhuysen, 1941; McKay, 
1942; Hiatt, 1948) . However, no F. subquadrata 
of any stage ever showed the 400% variation 
indicated by Hiatt (1948:163) for P. crassipes. 
Generally, during the moulting from the termi- 
nal prehard instar into the hard Stage I crab, 
and from the Stage I into the Stage II instar, 
there is little or no increase in size. 
Only two Stage II females were observed 
undergoing ecdysis. The first of these crabs 
showed no increase in carapace width although 
the abdomen became 20% wider. The other 
observed Stage II crab increased 10% in cara- 
pace width and 40% in abdomen width during 
the moult into the Stage III form. It is during 
this moult that a significant change occurs for 
the first time in the carapace-abdomen width 
ratio. Needham (1950) has discussed the quan- 
titative aspects of this allometric growth in 
P. pisum. 
The moult from the Stage III to the Stage IV 
instar, in six observed cases, was accompanied by 
an average 12.2% increase in carapace width 
and a 65% increase in the width of the abdo- 
men. During this moult occurs the greatest in- 
crease in abdomen width relative to the carapace 
width. 
